The present invention relates to a grease composition for constant velocity joints of motorcars, in particular, a sliding type constant velocity joints. More specifically, the present invention pertains to a grease composition for constant velocity joints, which can efficiently lubricate the portions of a constant velocity joint which are easily worn out and are liable to generate abnormal vibrations or the like; can effectively reduce the wear; can suppress any vibration; and can improve the useful life of the joint.
Recently, FF cars have rapidly increased, from the viewpoint of, for instance, reduction of the weight and the insurance of the accommodation space of a car, functional 4WD cars have likewise increased and accordingly, there have widely been used constant velocity joints (CVJ) suitable for these applications. An embodiment of a double offset joint (DOJ) is shown in FIG. 1, which is used as a sliding type joint among these CVJ's. In this DOJ, when the joint transmits a running torque, while it has a working angle, the joint undergoes complicated rolling and sliding motions at the fitting position of track grooves 3 of an outside wheel 1, track grooves 4 of an inside wheel 2 and balls 5, and the frictional resistance in the sliding portions of the joint generates a force in the axial direction. This force is referred to as "an induced thrust force". This DOJ is provided with track grooves 3 on the internal face of the outside wheel 1 at intervals of 60 degrees and therefore, the induced thrust force is generated 6 times per revolution.
If the cycle of such induced thrust forces is in agreement with the natural frequencies of, for instance, an engine, a car body and/or a suspension, a resonance of the car body is induced and this gives a disagreeable impression to the passengers of the car. Therefore, it is desirable to reduce the foregoing induced thrust force to a level as low as possible. In a packaged motorcar, inconvenience such as beating and stuffy sounds are generated during the high speed travelling of the motorcar due to the resonance. In addition, the lubricating conditions for the DOJs have increasingly become severer as the weight of the car has been reduced and the generating power of the engine has been increased, and correspondingly, the joint should be improved in the durability.
The conventional lithium-containing extreme-pressure grease, which comprises a sulfur-phosphorus-containing extreme-pressure additive, and a lithium-containing extreme-pressure grease composition, which comprises molybdenum disulfide, suffer from a problem concerning the vibration resistance. Moreover, these conventional extreme-pressure greases are greatly worn out under high contact pressure conditions and accordingly, they are also insufficient in the durability. Moreover, Japanese Un-Examined Patent Publication No. Sho 62-207397 discloses an extreme-pressure grease composition, which comprises, as an essential component, a sulfur-phosphorus-containing extreme-pressure additive comprising a combination of a molybdenum sulfurized dialkyl dithiocarbamate and at least one member selected from the group consisting of sulfurized fats and oils, sulfurized olefins, tricresyl phosphate, trialkyl phosphates and zinc dialkyl dithiophosphates. However, the extreme-pressure grease composition disclosed in this patent is still insufficient in the durability.